The activity of stem cells (neoblasts) in free-living flatworms are responsible for regeneration, growth and the continuous renewal of old tissues. The embryonic origin of the neoblasts is still unclear. This project aims at utilizing a 4D microscope (UGent, Nematology) to establish the cell lineage of polyclad embryos. An unambiguous and complete cell lineage of blastomeres in polyclads is the necessary first step to reveal the origin of totipotent stem cells in flatworms.